Polyolefins with molecular weight higher than 1×106 g/mol are generally termed as Ultrahigh Molecular Weight (UHMW) polyolefins. Ultrahigh Molecular Weight Polyethylene (UHMWPE) is known to demonstrate excellent toughness and high impact strength. This is mainly due to the presence of long and linear polymeric chains, as longer chains serve to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions.
UHMWPE with porous morphology and enhanced crystallinity complimented with reduced chain entanglement property is an example of another commercially desired polymer. Conventionally, it is prepared by ethylene polymerization catalyzed by metallocene or non-metallocene type homogeneous catalyst compositions, also known as single site catalyst compositions. The homogenous catalyst compositions mainly contain transition metal complexes such as complexes of Titanium (Ti), Zirconium (Zr), Hafnium (Hf) metals and the like. For activation of these homogenous single site catalyst compositions, relatively larger quantities of costly co-catalyst components such as methylaluminoxane are required. The use of relatively larger quantities of co-catalyst components results in less kinetic control and consequently reactor fouling by generation and deposition of polymer lumps on the reactor unit walls and stirrer shaft/paddle assembly. The use of a homogeneous catalyst composition, therefore, leads to increased polymerization cost.
Conventionally, heterogeneous Ziegler-Natta catalyst compositions based on titanium, magnesium and organo-aluminum co-catalyst have been developed. Since their inception till date, several modifications in the Ziegler-Natta catalyst compositions have been proposed in order to obtain specific properties in polymers, which are desired for specific applications.
The activity of these catalyst compositions to a great extent depends on the stability of Titanium (Ti+3)/(Ti+2) ion geometry, which is responsible for the growth of the polymer chain during the polymerization process. To improve the stability of Titanium (Ti+3)/(Ti+2) ion in the catalyst composition, electron donors are preferably added, either before the activation of the pro-catalyst component (internal electron donors) or after the activation of the pro-catalyst component (external electron donor). A variety of compounds have therefore been explored as internal and external electron donors for modification of heterogeneous Ziegler-Natta catalyst compositions.
Existing Knowledge:
U.S. Pat. No. 4,962,167 suggests a process for preparing an ultrahigh molecular weight polyethylene having narrow particle size distribution by polymerizing ethylene monomers in the presence of a catalyst composition comprising a solid catalyst component and an organometallic compound. The solid catalyst component as disclosed in the aforementioned US patent is obtained by contacting component A and component B, wherein component A is a reaction product of a magnesium dihalide and a titanium alkoxide and component B is a reaction product of an aluminum trihalide and a silicon alkoxide compound.
U.S. Pat. No. 6,559,249 suggests a process for preparing an ultrahigh molecular weight polyethylene with large bulk density and narrow particle size distribution by using an organometallic compound and a catalyst component. The catalyst component is prepared by reacting (i) a mixture of halogenated magnesium compound and an aluminum or a boron compound with alcohol to produce a magnesium compound, (ii) reacting the magnesium compound with an ester compound having at least one hydroxyl group and a silicon compound having an alkoxy group; and (iii) reacting the obtained reaction mixture with a titanium compound and a silicon compound. In the aforementioned US patent, esters and silane compounds are used as electron donors.
United States Patent Publication No. 2011159287 suggests a process for preparing ultrahigh molecular weight polyethylene by using a Ziegler-Natta catalyst composition that comprises a solid catalyst component obtained by reacting a magnesium compound, a titanium compound and an organo aluminum halogen compound, and a co-catalyst component. The use of external electron donors such as alcohols, ethers, esters, silanes and amines is also mentioned. However, the aforementioned United States Patent documents and patent application disclose the preparation of relatively dense ultrahigh molecular weight polyethylene with remarkably less porosity.
European Patent No. 0159110 suggests ultrahigh molecular weight polyolefin fine powders by using a specified Ziegler catalyst composition. The Ziegler catalyst used in the process of the aforementioned EP patent is prepared according to the teachings of Japanese Laid-open patent application 811/81 by reacting magnesium chloride and titanium chloride at low temperature and thereafter adding a small amount of monocarboxylate such as benzoate at high temperature. The aforesaid European patent document talks about the ultrahigh molecular weight polyethylene having a specified particle diameter and a specified particle size distribution and which is easily processable. However, the aforesaid European patent document is silent on the manufacturing of ultrahigh molecular polyethylene with improved porosity and crystallinity, and with flaky morphology.
Korean Patent No. 101161752 suggests a method for preparing an ultrahigh molecular weight polyethylene having high productivity and high apparent density (bulk density) by using a Ziegler-Natta catalyst composition. Though, the aforesaid Korean patent discloses the use of a heterogeneous Ziegler-Natta catalyst composition for the production of ultrahigh molecular weight polyethylene, it is silent on the production of ultrahigh molecular weight polyethylene having improved porosity and crystallinity, and flaky morphology.
Another Korean Patent No. 101144513 suggests an ultrahigh molecular weight polyethylene attributed with narrow molecular weight distribution which is prepared by polymerizing ethylene monomers in the presence of a heterogeneous Ziegler-Natta catalyst composition comprising a reaction product of a magnesium alcoholate, a titanium halide, an organoaluminum compound and an organo-silane compound. However, the aforesaid Korean patent document is also silent on the manufacturing of ultrahigh molecular weight polyethylene having improved porosity, crystallinity, and flaky morphology.
PCT Publication No. 2013087185 suggests a process for preparing ultrahigh molecular weight polyethylene with high powder bulk density by using a Ti—Mg based heterogeneous Ziegler-Natta catalyst composition. The ultrahigh molecular weight polyethylene as suggested in the aforesaid PCT document is dense in nature and characterized with poor crystallinity and porosity.
From the forgoing description, it is evident that though a number of records pertaining the production of ultrahigh molecular weight polyethylene using heterogeneous Ziegler-Natta catalyst compositions have been suggested, but none of these documents disclose the use of heterogeneous Ziegler-Natta catalyst compositions which when used in the production of polyolefins gives kinetically controlled polymerization reaction to provide polyolefins with significantly improved molecular weight along with flaky morphology, enhanced porosity and crystallinity, and lower to moderate bulk density, which are otherwise obtained by the use of expensive single-site homogeneous catalyst compositions.
Therefore, there is felt a need to provide a heterogeneous Ziegler-Natta catalyst composition which is an economic alternative to the existing expensive single site homogeneous catalyst system for the production of polyethylene having ultrahigh molecular weight and with porous and flaky morphology.
Objects:
Some of the objects of the present disclosure are described herein below:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a heterogeneous Ziegler-Natta catalyst composition with modified properties for producing polymers of ethylene monomers having ultrahigh molecular weight, increased intrinsic viscosity, enhanced porosity and crystallinity, lower to moderate bulk density and reduced chain entanglement.
Still another object of the present disclosure is to provide a heterogeneous Ziegler-Natta catalyst composition with better control on the polymerization process thereby obtaining a polymer of ethylene monomers with the desired bulk density and without reactor fouling.
Yet another object of the present disclosure is to provide a process for the preparation of heterogeneous Ziegler-Natta catalyst composition.
A further object of the present disclosure is to provide polymers of ethylene monomers having ultrahigh molecular weight, increased intrinsic viscosity, lower to moderate bulk density and having a unique flaky morphology with enhanced porosity, and reduced chain entanglement.
A still further object of the present disclosure is to provide a process for preparing polymers of ethylene monomers having ultrahigh molecular weight by using a heterogeneous Ziegler-Natta catalyst composition of the present disclosure.
A yet further object of the present disclosure is to provide a process for preparing polymers of ethylene monomers having ultrahigh molecular weight by using a heterogeneous Ziegler-Natta catalyst composition of the present disclosure that employs relatively lesser amounts of a co-catalyst component as compared to the conventional polymerization processes that employ single site catalyst compositions.
Another object of the present disclosure is to provide a process for preparing polymers of ethylene monomers having ultrahigh molecular weight, wherein the process is simple, cost-efficient and that eliminates the risk of polymerization unit fouling.
Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.